1. Field of the Invention
The present invention relates to a semiconductor device, a method of producing the same, and an apparatus for producing the same.
2. Description of the Related Art
With the progress in semiconductor integrated circuits in recent years, semiconductor elements having very many terminals (e.g., not less than 300 terminals) have been placed in the market. Accordingly, it has been strongly demanded to improve the technology for connecting the terminals (electrodes) of a semiconductor element to the terminals (electrodes) of a wiring board, and to reduce the cost.
Technology has been developed for connecting all the electrodes of the semiconductor element to the electrodes of the wiring substrate at one time by utilizing metal bumps. That is, metal bumps such as solder bumps or gold bumps are first attached to the electrodes of the semiconductor element, and the semiconductor element is pressed onto the wiring board, with its face directed downward, so that the metal bumps are joined to the electrodes of the wiring board and the electrodes of the semiconductor element are connected to the electrodes of the wiring board.
The conductors of the integrated circuit of a semiconductor element are formed of aluminum and, hence, the electrodes of the semiconductor elements are generally formed of aluminum. On the other hand, the conductors of a wiring board are composed of copper and, hence, the electrodes of the wiring board are generally formed of copper.
When solder bumps are to be used, a nickel layer and a titanium layer are formed on the aluminum electrodes of the semiconductor element and the solder bumps are joined to the electrodes having a composite structure of the semiconductor element, since solder joins poorly to aluminum. Thereafter, the semiconductor element is pressed onto the wiring board while being heated, so the solder bumps melt and spread on the electrodes of the wiring board, with the result that the solder bumps are surely connected to the electrodes of the wiring board.
When gold bumps are to be used, there is no need to form a nickel layer and a titanium layer on the aluminum electrodes of the semiconductor element unlike the case of using the solder bumps, since gold directly joins to aluminum. However, the gold bumps are attached to the electrodes of the semiconductor element in the form of stud bumps with projections, and, the semiconductor element is pressed onto the wiring board while being heated after the surfaces of the stud bumps are levelled, an electrically conducting adhesive is applied to the surfaces of the gold bumps, so that the gold bumps are connected to the electrodes of the wiring board via the electrically conducting adhesive. The electrically conducting adhesive comprises a mixture of a thermosetting resin and a metal filler mixed therein, and is thermally cured. Thereafter, the semiconductor element and the wiring board are sealed with a sealing adhesive (insulating resin) inserted therebetween.
When solder bumps are to be used, it is necessary to add the nickel layer and the titanium layer onto the aluminum electrodes of the semiconductor element, as described above but not all users of the semiconductor elements are necessarily allowed to apply the nickel layer and the titanium layer as desired, since application of the nickel layer and the titanium layer requires a special facility such as a vacuum chamber. Therefore, the solder bumps often cannot be used when a semiconductor element without a nickel layer and a titanium layer is purchased.
When an electrically conducting adhesive is applied to the gold bumps formed as stud bumps, on the other hand, the levelled surfaces of the stud bumps are not necessarily in parallel with the surface of the wiring circuit. Therefore, electric connection is not accomplished to a sufficient degree despite using the electrically conducting adhesive, and the reliability of connection remains low. Moreover, the materials are used in increased amounts, the steps of production are complex, and the heating must be continued until the resin is cured, hindering the productivity. Besides, in the case where the semiconductor element is defective or the mounting thereof is defective, the semiconductor element must be replaced by peeling the electrically conducting adhesive off the electrodes of the wiring board. However, it is difficult to peel it off after it is once thermally cured, since the electrically conducting adhesive contains a thermosetting resin. This makes it very difficult to repair the semiconductor element or the wiring board.